Please explain this SQL statement in English!

SELECT a.*,
       b.*,
       b.id AS user_id,
       a.i开发者_开发百科d AS group_id 
FROM groups a,
     users b 
WHERE a.group_leader_id = b.id

Thank you.

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Show all groups and only those users who are a leader of a group

SELECT a.*, b.*, 
b.id AS user_id, -- use an alias for users.id
a.id AS group_id -- use an alias for groups.id
FROM groups a, -- alias table groups as 'a'
  users b -- alias table users as 'b'
WHERE a.group_leader_id = b.id -- show only those users who are a leader of a group, and link them to that group

According to me, your query can be optimized to this:

SELECT a.*, b.*, b.id AS user_id, a.id AS group_id
FROM groups a 
  INNER JOIN users b ON a.group_leader_id = b.id

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SELECT a.*, b.*, b.id //select all columns from a, b and id from table b with alias user_id
AS user_id, a.id     // id from table a with alias group_id
AS group_id 
FROM groups a, users b  //from tables groups and users with aliases a and b
WHERE a.group_leader_id = b.id  //where group_leader_id from groups = id from users.

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This statement is a join between two tables groups and users on the group leader foreign key of groups.

So I will state it this way: Display the user information of the group leaders (with a redundant renaming of user_id and group_id).

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List all groups and their leaders.

SELECT 
    a.*, 
    b.*, 
    b.id AS user_id, 
    a.id AS group_id 
FROM 
    groups a
INNER JOIN 
    users b 
ON 
    a.group_leader_id = b.id

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It means pick out Group Leaders(doesn't say what details) from 2 tables where B has the list of Group Leaders, and A has the user's details.

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Give me all columns from group + all columns from users + rename b.id as user_id + rename a.id as group_id from all rows in groups where there is a match from groups to users.